1. Field of the Invention
The present invention relates generally to mobile radiocommunication systems.
2. Description of the Prior Art
Errors can occur in mobile radiocommunication systems when estimating data on reception, caused mainly by distortion of the transmitted signal due to multipath propagation, thermal noise and all the various sources of interference.
Redundancy is generally added to enable the receiver to retrieve the information bits transmitted, i.e. more than one bit is transmitted for each information bit. This technique is known as channel coding and the quantity of redundancy, defined as the ratio of the number of information bits to the number of bits transmitted, is referred to as the coding rate. The coding rate is therefore a number from 0 to 1, and the smaller the number, the greater the quantity of redundancy.
In the case of data transmission, it is necessary for each information bit to be received correctly. However, even for low coding rates and high transmission powers, errors are always possible on reception in the case of severe radio conditions. Another technique, known as the Automatic Repeat reQuest (ARQ) technique, is used in addition to the channel coding technique. It simply consists of retransmitting blocks of information bits that have not been received correctly by the receiver until they are received correctly. The proportion of blocks retransmitted is referred to as the BLock Erasure Rate (BLER) and depends to a significant degree on the coding rate (the BLER is an increasing function of the coding rate).
Thus the channel coding technique and the ARQ technique enable data to be received correctly in all radio conditions. However, a main drawback of these techniques is that they use some of the radio resources for a purpose other than increasing the net bit rate. The net bit rate is the bit rate obtained after deducting from the raw bit rate (the bit rate actually transmitted at the radio interface) everything that is not useful to the user, such as the redundancy introduced by the coding or the blocks not received correctly.
To maximize the net bit rate, it is therefore necessary to optimize the coding rate. This optimization is not a simple matter, since the relationship between the BLER and the coding rate is highly dependent on radio conditions. If radio conditions are poor, a low coding rate is preferable to avoid an excessively high number of retransmissions. In contrast, if radio conditions are good, a high coding rate is sufficient and produces a high net bit rate.
A plurality of coding schemes with different coding rates are necessary to obtain a net bit rate optimized for all radio conditions. They enable the coding rate to be adapted dynamically as a function of radio conditions. Thus a more rugged coding scheme (i.e. one having a lower coding rate) can be selected if radio conditions are degraded or, conversely, a less rugged coding scheme (i.e. one having a higher coding rate) can be selected if radio conditions improve. This technique is known as link adaptation.
For example, the GSM (Global System for Mobile Communication) standard specifies four coding schemes for the General Packet Radio Service (GPRS). The four coding schemes are denoted CS1 to CS4 and have coding rates from 0.5 to 1.
The link adaptation technique can also be used for dynamic adaptation of parameters of the system other than the coding rate, for example the modulation spectral efficiency (i.e. the capacity of the modulation to transmit a larger or smaller number of bits per symbol for the same allocated frequency band). Thus a more efficient but less rugged modulation scheme can be selected if radio conditions are degraded or, conversely, a more efficient but less rugged modulation scheme can be selected if radio conditions improve.
The link adaptation technique can also be applied to a combination of coding and modulation schemes. For example, the GSM standard specifies nine coding and modulation schemes MCS1 to MCS9 for the Enhanced General Packet Radio Service (EGPRS).
In the case of speech transmission, the link adaptation technique can also be applied to a combination of channel coding and speech coding (or source coding) schemes. This technique is known as the Adaptive Multi-Rate (AMR) technique.
Radio conditions are generally represented by a radio criterion, or quality indicator, such as in particular the raw Bit Error Rate (BER), the BLock Erasure Rate (BLER), the Signal-to-Interference Ratio (SIR), etc.
The theory of link adaptation is generally based on a system of N−1 ordered thresholds S1 to SN−1, where N is the number of coding and/or modulation schemes C1 to CN. These thresholds are called decision thresholds, and are defined so that, if the radio criterion used is between the thresholds Si and Si+1, (where i is from 1 to N−2), the coding and/or modulation scheme Ci is selected. If the radio criterion employed is below the threshold S1, the coding and/or modulation scheme C1 is selected. If the radio criterion employed is above the threshold SN−1, the coding and/or modulation scheme CN is selected.
The radio criterion is generally obtained by measurements referred to as radio measurements. The results of a plurality of successive measurements are generally averaged to obtain a more accurate value for the radio criterion.
However, using such means is not without its drawbacks. Adaptation is then slower (because the average does not depend only on the result of the most recent measurement, but also on the results of preceding measurements). It is then very dangerous to average over too long a time period because the adaptation may then not be fast enough if radio conditions are degraded rapidly, and performance may then be significantly affected, i.e. quality can be significantly degraded, and there is even a risk of the call being cut off.
One object of the present invention is to avoid such drawbacks.